This invention relates to a method of forming a passivation layer on a substrate. The invention relates also to a substrate comprising a passivation layer formed by the aforementioned method. The invention further relates to a method of removing a passivation layer from a substrate.
Many metals are susceptible to oxidation when exposed to or stored in atmospheric conditions. Oxidation of metals causes a metal oxide to form on the surface of the metal. The formation of surface metal oxides can have detrimental effects on the chemical, mechanical, optical and electrical properties of the metal. For these reasons, metal oxides are typically removed from metallic surfaces prior to forming an electrical connection. However, metal oxides may grow to beyond nanometre levels, and can be difficult and time consuming to remove. It is therefore desirable to prevent the formation of surface metal oxides.
A known approach for preventing the formation of surface metal oxides includes forming a passivation layer. A “passivation layer” is a protective film, layer or coating deposited on the surface of a substrate to suppress or inhibit chemical reactions, such as oxidation reactions, occurring at that surface. A passivation layer typically acts as an oxidation barrier layer, which inhibits oxidation of the underlying surface. In the case of a metallic surface, such as a copper surface, a passivation layer inhibits the oxidation of the metallic surface, and thereby prevents corrosion.
Some known passivation layers are permanent. For example, it is known to deposit alumina directly onto a metallic surface to suppress oxidation. However, removal of such a permanent passivation layer typically requires aggressive chemical etching, mechanical polishing or other processes which could damage other components of the substrate.
In some applications, such as PCB surface finishes or thermo-compression bonding, a temporary passivation layer is preferred. It is desirable to be able to selectively remove the passivation layer as required in preparation for subsequent processing steps. For example, it may be necessary to remove the passivation layer prior to forming an electrical connection in the fabrication of an electronic device.
When forming electrical connections, it is known to deposit a layer of organic solderability preservative (OSP) on a surface of a metal, such as copper, prior to soldering. The OSP layer inhibits oxidation of the metallic surface prior to soldering and acts as a passivation layer.
The OSP passivation layer must be of a minimum thickness to sufficiently suppress oxidation of the underlying metallic surface (usually several 100 nm), and can contaminate the substrate in subsequent processing steps. Known methods of depositing OSP layers typically result in the OSP layer having a non-uniform thickness across the substrate surface. Thick and/or non-uniform OSP layers may be difficult to fully remove and OSP residues may contaminate the substrate during subsequent processing steps.
It is desirable to decrease the thickness of a passivation layer, whilst preserving the oxidation resistant characteristics of the passivation layer. It is desirable for the passivation layer to have improved thickness uniformity. It is desirable to develop a passivation layer that can suppress or inhibit oxidation over a prolonged period, such as in storage, but which can be selectively removed from a substrate when desired. It is desirable to be able to selectively remove passivation layers without the need to use additional processing steps, such as aggressive chemical etching or mechanical polishing.
Alternative passivation coatings are also known, for example a passivation coating for micro-channel coolers is disclosed in R. W. Bonner III et al, Applied Power Electronics Conference and Exposition (APEC), 2012, 498-502. Bonner III et al. disclose a copper substrate where corrosion prevention is primarily controlled by plating layers of nickel and gold onto the copper substrate. A long-chain alkanethiol is used to form a self-assembled monolayer (SAM) on the polished gold surface, and subsequently an alumina layer is deposited thereon. Longer carbon chains are preferred because longer chained SAM molecules pack and organize better than shorter chain SAM molecules. The passivation coating of Bonner helps to aid erosion protection.